More than half of the world's population is chronically infected with the gastric pathogen H. pylori. Infection is usually acquired in early childhood and lasts for a lifetime with the majority of infected individuals remaining asymptomatic. H. pylori infection is the main cause of peptic ulcer disease (Kuipers et al. 1995), and is a significant risk factor for the development of gastric adenocarcinoma (Wilson & Crabtree 2007; Amieva & El-Omar 2008; Wroblewski et al. 2010). Furthermore, treatment of H. pylori is becoming increasingly difficult with the emergence of antibiotic resistance (Zullo et al. 2007; Graham & Shiotani 2008) and consequently alternatives to antibiotic treatment need to be identified. Elucidating the mechanisms of H. pylori persistence may lead to the discovery of novel drug targets and the development of alternative eradication therapies.
Genetics, including insertion mutagenesis, gene replacement and gene over-expression, has contributed tremendously to deciphering bacterial biological phenomena through the study of recombinant and mutant strains' phenotypes. This approach has been extensively used to characterize several H. pylori virulence factors, such as urease, CagA, VacA and flagella (Algood & Cover, 2006). Though initially useful, there is growing opinion in the field of bacterial genetics that the use of knockout mutants limits the study to complete a loss of function as this approach does not allow for investigating whether a specific gene or set of genes encoding virulence determinants is necessary to maintain the infection state once it has been established or whether these virulence determinants are necessary for the entire infection cycle (Gandotra et al. 2007; Liu et al. 2008). Moreover, severe phenotypes affecting bacterial growth are subjected to compensatory adaptation either at the physiological or genetic level or both. Therefore, a genetic tool, based on an inducible expression of the target gene, is better suited to constructing conditional knockouts for physiological and phenotypic studies. Of note, genetic studies based on the use of conditional knockouts enable the investigators to test for the temporal requirement of the target gene expression and their corresponding products, distinguish their role during the different steps of an infection, such as colonisation and maintenance of the infection. This approach is of particular importance for H. pylori infection which is a persistent and lifelong infection.
Recently a plasmid based inducible system based on the LacIq system in E. coli has been developed to study essential genes in H. pylori (Boneca et al. 2008). Unfortunately, the LacIq system does not repress gene expression efficiently and it requires large amounts of the inducer molecule to induce gene expression which makes it unfeasible to use as a tool to study the infection process in an animal model. Moreover, the DNA restriction barrier and DNA modification of H. pylori strains differ and prevent a systematic use of plasmids in this bacterium.
Thus, there remains a need for a genetic tool, based on an inducible expression of a target gene, in H. pylori that functions in vitro and in vivo. Moreover, there is a continuing need to develop eradication methods for H. pylori, especially the eradication of genetically modified H. pylori that can be used as a biological delivery vehicle for peptides and the like.